This invention relates to a target detecting radar system and, in particular, to a radar system for detecting targets having a radial velocity relative to the system.
A radar system, in general, includes a transmitter adapted to radiate short duration, high frequency, pulses of electromagnetic energy and a receiver responsive to the echo pulses returned when the radiated pulses impinge upon an object. The information provided by the echo pulses is derived and presented on an indicating device.
A radar system which is adapted to distinguish between fixed objects and moving objects and which applies to the indicating device signals representing moving targets only is referred to as a moving target indicating (MTI) system. Distinction between moving and fixed targets is normally attained by utilizing the Doppler effect, i.e., the phase change of the echo with respect to the transmitted pulse due to the radial velocity of the target with respect to the radar system. In contrast, the phase relationship between a transmitted pulse and an echo resulting from a fixed target is the same for successive pulses. The fact that for successive pulses the relative phase of the transmitted pulse and the echo varies when the target is moving, and is constant when the target is fixed, provides a means for distinguishing between fixed and moving targets.
One type of prior art moving target indicating system periodically transmits a group of two pulses at a predetermined repetition frequency, the Doppler phase-shift produced between the two pulses of a group being utilized to detect moving targets. An important advantage of this type of system is the effective elimination of so-called "blind speeds" by closely spacing the pulses of a group. "Blind speeds" result from the failure of a moving target indicating system to detect phase changes of whole cycles from objects which move toward or away from the radar system a distance equal to an integral number of half-wave lengths of the transmitted radio frequency energy during the interval between pulses.
One such moving target indicating radar system is disclosed in U.S. Pat. No. 3,225,349 issued to Robert C. Thor on Dec. 21, 1965. In this system, a radar transmits an upswept frequency pulse for a time T. After a time .tau.-T, the radar transmits another pulse. On the second pulse the radar changes the direction of the frequency sweep. The cycle is repeated after a delay during which echos are received and pulse-compressed. Two receiver channels in the radar contain respective matched filters. The first filter compresses echoes of the first pulse, while the second filter compresses echoes of the second pulse. Stationary target echos appear at the same range on successive transmissions, whereas radially moving targets appear at different ranges on successive transmissions. Because of this difference in behavior, echoes from the first pulse can be subtracted from echos from the second pulse to cancel non-moving target echos, without cancelling moving targets echos.
This system suffers from the following disadvantage. Since the interpulse period of the radar is short, long-range echos of the first pulse arrive at the radar at the same time as short-range echos of the second pulse. Thus, the filters in the receiver each act on both matched and mismatched signals. In the presence of distributed clutter, the compressor limiter in the two receiver channels will not suppress the mismatched component and the moving target indicating radar system will not provide significant clutter cancellation or subclutter visibility.